Solvent Properties of Carbon Dioxide

The RESS process relies on the solvent properties of carbon dioxide. Because CO2 is a nonpolar molecule, this process will be mainly efficient and interesting for micronizing nonpolar molecules. For this reason, a preliminary study on the solubility of the compounds with pressure and temperature is necessary. As usual, the solvent polarity can be modified and enhanced by adding to the supercritical CO2, small quantities of an organic cosolvent. This is primarily because the solvent power of an SCF is strongly dependent on its density, which can be adjusted by small variations of pressure and temperature (11). 

According to the duPont brochure, fluoroether oil is not soluble in hexane. The solvent properties of carbon dioxide are often compared to those of hexane (Hyatt, 1984). Hyatt examines supercritical carbon dioxide from the perspective of an organic chemist, and his conclusions are a good summary of his findings. He states that... 

Kho, Y.W., Conrad, D.C. and Knutson, B.L. (2003) Phase equilibria and thermophysical properties of carbon dioxide-expanded fluorinated solvents. Fluid Phase Equilibria, 206 (1-2), 179-193. 

Aschenbrenner O, Styring P (2010) Comparative study of solvent properties for carbon dioxide absorption. Energ Environ Sci 3(8) 1106-1113... 

In order to examine a little more closely the range of solvent conditions in which we are interested, it is convenient to refer to the reduced pressure versus reduced density diagram for carbon dioxide shown in Figure 1.1. Although the figure is based on the properties of carbon dioxide, the dependence of reduced density on reduced temperature and pressure is qualitatively correct for other non-polar and slightly polar solvents also. 

The key physical property of carbon dioxide is its excellent solvent properties (see Table 2) for many nonpolar organic compounds. Like most solvents, the solvent properties of C02 improve as the pressure and temperature increase. In cleaning, we rely upon the liquid phase solvent properties. It is important to note that thermodynamically, liquid carbon dioxide is unstable at room temperature and atmospheric pressure but this thermodynamic condition only refers to equilibrium states, not non-equilibrium states. 

Preparation, characterization, and properties of silicon-containing triorganotin fluorides, both symmetrical and unsymmetrical, were investigated. It was observed that introduction of a trimethylsilyl group in the alkyl chain results in a considerable enhancement of solubility in various nonpolar solvents including dense carbon dioxide. 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Lu, J., Liotta, G.L., Eckert, G.A., Spectroscopically probing microscopic solvent properties of room-temperature ionic liquids with the addition of carbon dioxide, /. Phys. Chem. A, 107, 3995-4000, 2003. 

Several liquids and gases can be brought into the supercritical phase. Different solvents can be selected as extraction media for use in analytical-scale SFE. Carbon dioxide is most commonly used as an SFE medium because of its desirable properties and easy handling it is relatively inexpensive and commercially available at a purity grade acceptable for most analytical applications. Another advantage of carbon dioxide is that the polarity can easily be adjusted by adding modifiers such as methanol to the supercritical fluid or the extraction vessel. 

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